Introduction
Nowadays every pc in an organization is somehow connected to a network. The most common type used is LAN (Local Area Network). Since LAN’s need a lot of cable work and restricted mobility, WLAN is becoming much popular now. Continuous research is going on to develop high speed and low cost wireless networks.
Wireless local area network WLAN is one such solution which paved the way for the futuristic mobile enterprise where ‘Mobility’ is a prime factor. WLAN’s compared to wired Ethernets deliver the same services at an affordable price and which eases a network administrator of taking care of all those confusing cabling, and eases a user to use his laptop to connect to the network anywhere in the office.
In this project, ‘Mobile Communication and Signal Processing ‘, we have learned the basic concepts o WLAN, IEEE standards of WLAN and an introduction to Sensor Networks and RFID. We measured the signal strength of an Access Point at various distances and estimated the power distance gradient.
WLAN (Wireless local area network)
1.1 Introduction:-
As the name suggests WLAN is a network connected wirelessly communicating through signals via various communicating protocols. 802.11a is a family member of IEEE 802 used for local area networks. It is primarily focused on the physical and data link components.
There exist various WLAN standards. According to IEEE standards group figure 1 show different types of WLAN and their parameters accordingly.
802.11b / 802.11a / 802.11gMax bit rate, raw
net / 11Mb/s
5.5 Mb/s / 54 Mb/s
22-26 Mb/s / 54 Mb/s
17-22 Mb/s
Frequency band / 2.4GHz / 5 GHz / 2.4 GHz
Range @ max rate / 57 m / 12 m / 19 m
Current@3V / 300 mA / 550 mA / 400 mA
Unit Cost / 100% / 120% / 110%
Coverage cost (@max rate) / 100% / 2000% / 500%
Modulation Scheme / DSSS / OFDM / OFDM above 20Mbps, DSSS below 20Mbps
802.11 Network consists of four major components:-
Distribution system: - It is a logical component of 802.11. It is used to send the data from source to destination. The access points are connected to each other covering a large area to track the movements of mobile stations.
Access points: - It performs the wireless-to-wired bridging function. It converts to any type of frame for delivery to outside world.
Wireless Medium: - Two wireless mediums were standard namely the radio frequency (RF) and the infrared (IR).
Stations: - Stations are computing devices with wireless network interfaces. They can be better operated laptops or handheld computers.
1.2 Types of network architecture :-
The 802.11 has mainly 3 types of networks divided according to their configuration and extensibility of the network.
Basic Service Set (BSS): It forms the fundamental core of the network architecture for all the network types; a group of stations communicating with in a particular area is called a basic service area. In a BSS a station can communicate with every other station in that particular area.
The network architecture is divided mainly three types according to the service area.
(a) Independent Network: -
Stations in an Independent Service area (IBSS) communicate directly with each other and these must be with in direct communication range. The smallest possible 802.11 network is an IBSS with two stations. Because their short lived nature they are referred as adhoc BSS’s or adhoc networks.
Ad hoc network
(b) Infrastructure BSS: -
In this type of networks there exist an access point through which all the data from and mobile station is handled. This is actually efficient way unlike in Independent BSS this is handled by the access point in the infrastructure BSS, a mobile station has to connect the access point in order to communicate with the neighboring mobile station. Here the AP acts as a router between different mobile stations.
Access Point
(c) Extended service Set:- This is created by linking multiple BSS with a back bone network. Here the access points are configured to the same ESS and acts as bridges. The mobile stations with in the same ESS can communicate directly to each other although they belong to different BSS.
1.3 802.11 MAC Sublayer :
This layer along with Logical link control sub layer (LLC) provides a link between higher level layers and the physical layer of an OSI model.
it takes care of transmitting and receiving the frames decoding frame header, decrypting data, decoding address, decoding frame check, decoding frame check, decoding sequence control, decoding duration id etc.
Properties of wireless links are
- Low received signal quality
- High probability of bit errors
- Effects of noise and interference
Ex: microwave oven, cordless phones, and other 802.11 networks
- Low transmitted power: limited battery life
- Multipath fading
The above are the challenges of a MAC layer.
MAC Access Modes :
The MAC employs various co ordination functions namely
- PCF : point co ordination function - it employs a contention free services where the access point holds the point co coordinators and the stations transmit frames at shorter intervals it is a kind of polling where the stations have to wait for their turn this type of functions are not really implemented in a real life scenarios.
- DCF : Distributed Co ordination function – it uses Carrier Sense Multiple Access with collision avoidance (CSMA/CA) access mechanism for channel sensing to avoid collisions, in some cases it also employs CSMA/CA with CTS/RTS
Carrier Sensing :
Carrier sensing is used to determine if the medium is free
Any wireless device which uses the wireless medium to send frames from and through must be aware of the following basic things which they have to perform.
Carrier sensing is in two ways.
- Physical: this is done at the physical layer it is costly to implement and not so efficient because it depend on medium and modulation and it suffers from hidden node problem.
- Virtual: this type of sensing measures
- Time interval of the received frame means how long time it takes to send a frame and receive an acknowledgement.
- To reserve the channel for certain amount of time.
- The frames have a duration field that is used to reserve the medium.
- During this time the other nodes refrain from transmission.
Positive Acknowledgement:
It sets the rule that every sent frame must have a positive acknowledgement.
CSMA/CA :
This scheme is widely popular in Ethernets, whenever a transmitter want to send a frame it sense the channel both physical and virtual schemes are employed, if the channel is free it send the frame if it detects any activity then it waits until the channel is free.
There is however a problem with this type of access mode, when the channel is free csma/ca send the entire frame at one go and wait until it receive the acknowledgement, if in case it does not receive any acknowledgement it enters backoff phase in which it keeps increasing the time to resend the packet.
CSMA/CA suffers from hidden node problem.
CSMA/CA with RTS/CTS :
This is same as CSMA/CA except it also includes Request to Send (RTS), Clear to Send (CTS).
NAV :
Virtual carrier sensing is provided by NAV, most frames carry a duration field which is actually used to reserve the medium, NAV serves as the timer which indicate the amount of time the channel will be used. When the stations broadcast RTS/CTS which actually hold the duration information all the other stations which receive the respective RTS/CTS frames reset their timers according to the received frames time and start counting down to zero.
sender
SIFS time
SIFS reciever
time
DIFS NAV
time
access to medium deferred contention window
Using the NAV for virtual carrier sensing
RTS:
Whenever a transmitter want to send a frame it first send a RTS frame with the estimated time to be taken i.e. the time it takes to send the frame + RTS ,this tells all other stations about the channel will be reserved , this procedure has many advantages,
- It reserves the channel.
- when the transmitter station send an RTS all the neighbouring stations including the destination station know that there is a station which want to use the channel
CTS:
When the receiver station receive the RTS frame it responds with the CTS frame with the (estimated time taken for the sender to send the frame -RTS)+ CTS+some interframe spacing. it is broadcasted to its neighbouring stations and also the receiver.
These methods mainly solve the hidden node problem which is a big bottleneck in wireless transmission creating collisions
1.4 Operations in 802.11
List of operating services in an 802.11 networks:
(a) Authentication:- This ensures security by establishing the identity of the station before association.
(b) De authorization:- It terminates an authenticated relationship and the current association.
(c) Integration:- It is provided by distributing system to communicate with a non-IEEE 802.11 (or) IEEE LAN out side the wireless network.
(d) Privacy:- It performs to provide roughly equivalent privacy to wired networks by encrypting frames as they travel across the 802.11 air interface.
(e) Data delivery:- Data is delivered to the recipients from the access point. We have some more operations such as Distribution services, Re-associations, associations etc.
Wireless channel modeling:- Channel modeling is used to
Determine the optimum location for installation of antennas and analyze the interface of the system.
Types of channel models:-
Troposcatter:- Troposcatter communication link is used in military operations for long distance communications. Here the signals are directed to wards the troposcatter layer of the atmosphere, the incident wave is scattered and some of the scattered signal energy reaches the receiver.
Microwaves LOS:- It is widely used in nation wide networks for terrestrial communication. Here the antennas are aligned to provide LOS communications. The signal propagated in this way is some times affected by the atmospheric changes creating a multipath condition.
Indoor area:- In this type phase and amplitude of the signal arriving on each from different paths are related to the path length and condition of the path. In indoor environment the multipath is caused by the reflection from the automation factor, walls, ceiling and objects with in environment.
Chapter 2
Sensor networks
2.1 Introduction:-
In the present world , where wireless communications play a vital role, there sensor networks are gaining much popularity. The basic idea behind these types of networks is low-cost, low power and processing or data acquisition. The development of sensor networks requires different research areas like sensing, communication and computing where it includes hardware, software, communications and computing algorithms.
Sensor networks consist of large number of nodes which communicates with in themselves for various tasks such as sensing, communication and computing.
2.2 Design factors
The design of sensor networks are influenced by many factors mainly including fault tolerance, scalability, production cost, operating environment and power consumption etc.
Fault tolerance:-
Sensor node will fail due to lack of power, physical damage and environment effects. The failure of nodes should not change the overall task of the sensor networks. This is modeled using poison distribution, the probability of number of failures in time interval (0, t) is
Rk(t) = e-lkt
Where lk is failure rate of sensor node and‘t’ is time period.
Scalability:-
It requires large number of sensor nodes to study a particular event. There are some situations where the nodes are few or in plenty in close proximity. In such situations the sensor networks should utilize or optimize the network accordingly to the situation.
Production costs:-
Sensor networks consists of a large number of sensor nodes, the cost of a single node is very important to calculate the overall cost of the network.
Environment:-
Operating environment depends on the application designed to use such a system and the scenario where these nodes are placed. They may be used inside machinery, a space station monitoring plants, a machinery building factory, or under water.
Power consumption:-
The main task of a sensor node in a sensor field is to detect events, perform fast local data processing and then transmit the data, power consumption, which can be divided into three parts sensing, communication and data processing.
2.3 Applications and Technical challenges:-
(a) Infrastructure security:- Sensor networks can be used fro infrastructure security to stop the terrorist activities. All the private, public organizations and large scale companies have the important role and need to the common man. Anti-social elements tend to disturb the life of common man thus we have the importance of this fixed sensor networks which can detect the possible threats.
(b) Industrial sensing:- These sensors will check the life of the machines used in the industry it is going to measure the vibrations, the ware and tare, lubrication level etc. The infrastructure sensor networks are very helpful for the industrial safety.
(c) Traffic control:- It gives information about how to control the traffic jams. Some sensor networks are buried in the ground, some of the sensors are the video cameras, some sensors are placed on the vehicle so that there is a communication between vehicle to sensor nodes and different sensor nodes which probably helps to determine the necessary requirements for the traffic control.
Technical challenges:
a) Adhoc discovery – to know the identity and location of the neighbouring nodes.
b) Rating and n/w control: connectivity
c) Signal and information processing: degree of information sharing between the node and collection of data from other nodes.
d) Security: The important and main aspect to prevent the use of sensors by enemies against intrusion and spoofing.
e)
Chapter 3
RFID
3.1 Introduction
RFID’s are called Radio Frequency Identification, a technique, similar in theory to the bar code identification. These devices operate at radio frequencies typically at 134.2 kHz, 13.56 MHz and at higher frequencies. Based on the various applications and uses they have different technical models and designs.
RFID systems can be used just about anywhere, from clothing tags to missiles to pet tags to food – anywhere that a unique identification system is needed. It contains of antenna and a transceiver which read the radio frequency and transfer the information to a processing device and a transponder or tag, which is an integrated circuit containing the RF circuitry and information to be transmitted. RFID is also called as dedicated short range communication (DSRC).